[GitHub] [spark] allisonwang-db commented on a change in pull request #33284: [SPARK-36063][SQL] Optimize OneRowRelation subqueries

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allisonwang-db commented on a change in pull request #33284:
URL: https://github.com/apache/spark/pull/33284#discussion_r670026456



##########
File path: sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/optimizer/subquery.scala
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@@ -701,3 +702,43 @@ object RewriteLateralSubquery extends Rule[LogicalPlan] {
       Join(left, newRight, joinType, newCond, JoinHint.NONE)
   }
 }
+
+/**
+ * This rule optimizes subqueries with OneRowRelation as leaf nodes.
+ */
+object OptimizeOneRowRelationSubquery extends Rule[LogicalPlan] {
+
+  object OneRowSubquery {
+    def unapply(plan: LogicalPlan): Option[Seq[NamedExpression]] = {
+      CollapseProject(EliminateSubqueryAliases(plan)) match {
+        case Project(projectList, _: OneRowRelation) => Some(stripOuterReferences(projectList))
+        case _ => None
+      }
+    }
+  }
+
+  /**
+   * Rewrite a subquery expression into one or more expressions. The rewrite can only be done
+   * if there is no nested subqueries in the subquery plan.
+   */
+  private def rewrite(plan: LogicalPlan): LogicalPlan = plan.transformUpWithSubqueries {

Review comment:
       The reason why we need to check subqueries is to deal with nested subqueries:
   ```
   Project [scalar-subquery [a]]
   :  +- Project [scalar-subquery [b]] <-- collapsible if transform with nested subqueries first
   :     :  +- Project [outer(b) + 1]
   :     :     +- OneRowRelation
   :     +- Project [outer(a) as b]
   :         +- OneRowRelation
   +- Relation [a]
   ```
   A subquery's plan should only be rewritten if it doesn't contain another correlated subquery. If we do not transform the nested subqueries first, we will miss out cases like the one above.




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